Segment reporting - Narrative (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Revenue, Major Customer [Line Items]
Number of operating segments			2
Number of contracts	3	5	2
Gross Premiums Written
Revenue, Major Customer [Line Items]
Concentration risk, percentage	100.00%	100.00%	100.00%	100.00%
Contract One | Contract Concentration Risk | Gross Premiums Written
Revenue, Major Customer [Line Items]
Concentration risk, percentage	20.50%	22.70%		18.00%
Contract Two | Contract Concentration Risk | Gross Premiums Written
Revenue, Major Customer [Line Items]
Concentration risk, percentage	11.60%	17.00%	12.80%	18.00%
Contract Three | Contract Concentration Risk | Gross Premiums Written
Revenue, Major Customer [Line Items]
Concentration risk, percentage	10.00%	14.60%		11.50%
Contract Four | Contract Concentration Risk | Gross Premiums Written
Revenue, Major Customer [Line Items]
Concentration risk, percentage		14.10%
Contract Five | Contract Concentration Risk | Gross Premiums Written
Revenue, Major Customer [Line Items]
Concentration risk, percentage		14.00%